In recent years, interest in using microwave signals for applications in many industrial and medical settings has grown dramatically. Some of these applications include using microwave power for heat treating various materials, polymer and ceramic curing, sintering, plasma processing, and for providing catalysts in chemical reactions. Also of interest is the use of microwaves for sterilizing various objects. These applications require electromagnetic exposure chambers or enclosures with relatively uniform power distributions. Uniform power distributions within the chambers help to prevent "hot" or "cold" spots which may cause unnecessary destruction or waste of sample material. Some of these applications also require that substances be passed through--rather than simply placed in--microwave chambers.
The prior art includes various attempts to achieve more uniform exposure of samples to microwave fields. Commercial microwave ovens utilize "mode stirrers", which are essentially paddle wheels that help create multiple modes within a microwave chamber. Many researchers have analyzed the use of multimode chambers for increasing uniformity of exposure. See Iskander et. al, FDTD Simulation of Microwave Sintering of Ceramics in Multimode Cavities, IEEE MICROWAVE THEORY AND TECHNIQUES, Vol. 42, No. May 5, 1994, 793-799. Some have suggested that the limited power uniformity achievable by mode stirring at a single frequency may be enhanced by using a band of frequencies. See Lauf et. al, 2 to 18 GHz Broadband Microwave Heating Systems, MICROWAVE JOURNAL, Nov. 1995, 24-34.
Designers have focused on multimode cavities because single mode cavities are seen as inevitably producing a field with a very limited peak region. See Lauf at 24. But multi-mode cavities have yet to produce highly uniform fields across an entire cross section of a microwave chamber. Although these cavities result in a plurality of field peaks across a chamber, they have many hot and cold spots. For every energy peak in such a cavity, there is a corresponding valley. Attempts to fill in these valleys with the peaks of waves operating at different frequencies creates other problems. The use of large bandwidth swept frequency generators makes the apparatus expensive and inefficient, since power at some frequencies will be reflected back to the source.
The possibility of a dielectric slab-loaded structure that elongates the peak field region in a single mode cavity has been long--but not widely--recognized See A. L. Van Koughnett and W. Wyslouzil, A Waveguide TEM Mode Exposure Chamber, JOURNAL OF MICROWAVE POWER, 7(4) (1972), 383-383. Koughnett and Wyslouzil disclosed the theoretical existence of a slab-loaded chamber supporting TEM-mode propagation. However, they did not disclose a chamber with openings that facilitate the introduction of substances for exposure to a relatively uniform electromagnetic field.
A slab loaded structure has been used in a few limited applications as a microwave applicator. Specifically, a slab loaded guide has been tested for radiating microwaves into tissue-like samples. See G. P. Rine et. al, Comparison of two-dimensional numerical approximation and measurement of SAR in a muscle equivalent phantom exposed to a 915 MHz slab-loaded waveguide, INT. J. HYPERTHERMIA, Vol. 6, No. 1, 1990, 213-225.
Although used in the context of microwave applicators, dielectric slabs have not been pursued in the context of microwave chambers. In fact, most of the prior art accepts a nonuniform field as a given and attempts to achieve even heating by other means. For example, a recent sintering patent directed itself at wrapping samples in an insulating "susceptor" to uniformly distribute energy to samples placed in a nonuniform microwave field. U.S. Pat. No. 5,432,325.
Aside from the problems associated with field uniformity, use of microwaves in some applications has been limited by concerns over radiation. Chokes that prevent the escape of electromagnetic energy from the cracks between two contacting surfaces are well known in the art. Particularly well known are chokes designed for microwave oven doors and wave guide couplers. See, e.g., U.S. Reissue Pat. No. 32,664 (1988). However, many potential applications require a cavity that has access points that are continually open. For these applications, substances need to be passed through, rather than placed in, the cavity. The prior art has not fully explored the use of choke devices to prevent energy radiation in structures that have continually open access points.
In the context of microwave applicators, continually open access points pose no problem. The goal of such devices is to radiate energy. However, in the context of microwave chambers, where the goal is to energize only the space inside the chamber, continually open access points present potentially harmful sources of radiation. The problem of radiation through open access points is magnified when the substance being passed through the chamber has any conductivity. Such conductive substances (e.g., any ionized moisture in paper that is passed through a chamber for drying) can, when passed through a microwave chamber, act as an antenna and carry microwaves outside the cavity.
In many important areas, microwave systems are not in use at all due to the problems posed by nonuniform fields and the need for continually open access points. For example, medical tubing is still sterilized either by chemical baths or by electron beam radiation. However, microwave methods have distinct advantages over electron beam (UV) methods. Microwaves are less likely to structurally damage the tubing. Also, microwaves can achieve greater depth of penetration than UV radiation. Therefore, medical tubing is more permeable to microwaves than to UV radiation. Furthermore, microwaves can kill organisms and help destroy and remove debris throughout the tubing. UV radiation can only kill organisms at or near the tubing's surface but not effectively remove debris. Yet microwave structures are not currently employed for pre-use sterilization of medical tubing.